There exist methods for converting lignocellulosic biomass into fermentable C5 and C6 sugars. Several of these methods first produce oligomers of the C5 and C6 sugars, which are then hydrolyzed to form fermentable streams of monomers of C5 and C6 sugars. Problems exist with current methods, including, inter alia, that a certain portion of the C5 oligomers is lost and not converted into C5 monomer under the processing conditions. To counter this problem, the methods may be performed to drive the hydrolysis of the biomass towards monomer. However, these more stringent conditions often lead to degradation products, such as acids that inhibit fermentation. It would, therefore, be beneficial to develop methods that avoid this tradeoff to maximize monomer formation and to minimize the formation of degradation products.
Others have attempted to circumvent the problems above. For example, U.S. Pat. No. 5,125,977 is directed to a two-stage dilute acid prehydrolysis process on xylan containing hemicellulose in biomass is effected by: treating feedstock of hemicellulosic material comprising xylan that is slow hydrolyzable and xylan that is fast hydrolyzable under predetermined low temperature conditions of 90-180° C. under ambient pressure with a dilute acid for a residence time sufficient to hydrolyze the fast hydrolyzable xylan to xylose; removing said xylose from said fast hydrolyzable xylan and leaving a residue; and treating said residue having a slow hydrolyzable xylan with a dilute acid under predetermined high temperature conditions of 160-220° C. under ambient pressure for a residence time required to hydrolyze said slow hydrolyzable xylan to xylose. However, applicants are concerned with degradation products produced under much higher temperatures and pressure conditions, which causes more significant issues with control of the process, especially since the residence times are very short, relative to those typically used (7-9 minutes and 3-5 minutes, respectively, in the first and second stages of U.S. Pat. No. 5,125,977). Unexpectedly, applicants have learned how to control processes employing hot compressed water at much high temperatures used in conventional methods and at very short residence times relative to conventional methods to achieve a balance of improving the xylose yield without producing undesirable degradation products. The methods and compositions of the present invention are directed toward these, as well as other, important ends.